High salinity and high arsenic concentrations are the two major natural water-quality problems in parts of the Willamette Basin. Ground water with high salinity is commonly encountered in the marine sedimentary rocks in the Coast and Cascade Range foothills and the valley margins adjacent to these areas. The salt in these rocks most likely originates from sea water that was incorporated into the sedimentary rock when it was originally deposited millions of years ago. This saline water mixes with shallow ground water in certain areas where deep regional ground water is upwelling. Saline water is also encountered in the Columbia River Basalt Group aquifers where they overlie marine sedimentary rocks. Excessive pumping of basalt aquifers is known to induce the flow of saline water from the underlying marine sedimentary rock and increase the salinity of ground water.
Ground water with elevated arsenic concentrations is commonly encountered in the southern Willamette Basin. High arsenic concentrations are thought to coincide with the presence of volcanic rocks of one or two geologic formations. A systematic analysis of the distribution of arsenic in ground water has not been done, nor has the exact mineral source of the arsenic been identified.
In addition to high salinity and arsenic concentrations, elevated levels of radon and phosphorous are also encountered locally in the Willamette Basin. Ground water enriched with phosphorous in the Tualatin, Pudding, and Yamhill River basins is thought to contribute to eutrophic conditions as well as other water-quality problems.
The policy stated in the Willamette Basin Report related to ground-water quality is to "ensure safe municipal and domestic ground-water supplies by participating with DEQ [Oregon Department of Environmental Quality] and the Oregon State Health Division in a formal monitoring program to document changes in [water] quality and provide data for aquifer management." Information on the geology and hydrology of water with high salinity, arsenic, radon, and phosphorous concentrations would help in implementation of this policy.